Recently, a nonaqueous electrolyte battery such as a lithium ion secondary battery has been actively researched and developed as a high energy-density battery. The nonaqueous electrolyte battery is expected to be used as a power source for hybrid vehicles, electric cars, an uninterruptible power supply for base stations for portable telephone, or the like. For this, the nonaqueous electrolyte battery is desired to have a high energy density as well as to be excellent in other performances such as rapid charge-and-discharge performances and long-term reliability. For example, a nonaqueous electrolyte battery enabling rapid charge-and-discharge not only remarkably shortens a charging time but also makes it possible to improve performances related to motivity and to efficiently recover regenerative energy from motivity, in a hybrid vehicle or the like.
In order to enable rapid charge-and-discharge, electrons and lithium ions must be able to migrate rapidly between the positive electrode and the negative electrode. However, when a battery using a carbon-based negative electrode is repeatedly subjected to rapid charge-and-discharge, dendrite precipitation of metal lithium occurs on the electrode, raising the fear as to heat generation and fires caused by internal short circuits.
In light of this, a battery using a metal composite oxide in place of a carbonaceous material in the negative electrode has been developed. Particularly, in a battery using titanium oxide as the negative electrode active material, rapid charge-and-discharge can be stably performed. Such a battery also has a longer life than those using a carbonaceous material.
However, titanium oxide has a higher potential based on metal lithium than the carbonaceous material. That is, titanium oxide is nobler. Furthermore, titanium oxide has a lower capacity per weight. Therefore, a battery using titanium oxide as the negative electrode active material has a problem that the energy density is lower. Particularly, when a material having a high potential based on metal lithium is used as a negative electrode material, a battery using the material has a lower voltage than that of a conventional battery using a carbonaceous material. Therefore, when the battery is used for systems requiring a high voltage such as an electric vehicle and a large-scale electric power storage system, the battery has a problem that the battery series number is increased.
The potential of the electrode using titanium oxide is about 1.5 V based on metal lithium and is higher (nobler) than that of the negative electrode using carbonaceous material. The potential of titanium oxide is due to the oxidation-reduction reaction between Ti3+ and Ti4+ when lithium is electrochemically inserted and extracted, and is therefore limited electrochemically. It is therefore conventionally difficult to drop the potential of the electrode to improve the energy density.